1. Field of the Invention
The present invention relates generally to an imaging device and, more specifically, to an image sensor containing row drivers of enhanced reliability configured to transmit a signal with a higher than nominal operating voltage.
2. State of the Art
An image sensor is a semiconductor device with the capacity to convert an optical image into an electrical signal. Image sensors are used in a variety of imaging applications including medical products, navigational equipment, and consumer products such as digital cameras and cellular phones.
Image sensors include an array of photo-sensitive devices such as photodiodes or photo-transistors fabricated on, for example, a complementary metal oxide semiconductor (CMOS) device. Each photo-sensitive device is sensitive to light in such a way that it can create an electrical current that is proportional to the intensity of light striking the photo-sensitive device. The overall image captured by an image sensor is comprised of multiple pixels in an array, arranged in rows and columns, such that each pixel detects the light intensity at the location of that pixel. Each row in the pixel array may include a row driver configured to send a plurality of signals to an identified pixel or row of pixels. For example, if an image sensor includes one thousand rows, one thousand row drivers may be required. The plurality of signals may include a row selection signal, a reset signal and a transfer signal. Upon receipt of a signal, the pixel array may perform various functions on the identified pixels including selecting, resetting, or transferring the charge of one or more identified pixels in an identified pixel row.
Currently, for improved performance, it is common practice for the reset and transfer gate pixel signals to be amplified by a voltage boost which is greater than an operating voltage supply. The term “boost” and its derivatives are used throughout this specification to indicate a voltage or a generation of a voltage which is higher than the operating voltage supply.
As stated, increasing the voltage on the pixel signals improves the performance of the imaging sensor. One enhancement involves the gating operation of the row select, reset, and transfer transistors. In particular, by increasing the voltage to one or more of the gates of the row select, reset, and transfer transistors, it is possible to avoid pixel to pixel fabrication differences in electrical characteristics of these transistors. Additionally, by boosting the voltage of the reset signal, the dynamic range of the sensor pixels can be increased without increasing the supply voltage. Dynamic range refers to the range of incident light that can be accommodated by an image sensor in a single frame of pixel data. However, boosting the voltage can cause reliability problems with the transistors. In particular, a boosted voltage can cause terminal voltages of the transistor to be greater than the operating voltage supply of the system. Examples of terminal voltages may include a gate-to-source, a gate-to-drain, or a drain-to-source voltage. Transistors operated at voltages higher than the normal operating voltage supply have a far greater failure rate due to lower device reliability than transistors operating at, or lower than, the operating voltage supply.
Based on the foregoing, there remains a need for an imaging device containing row drivers which are capable of overcoming the disadvantages delineated above. In particular, a need exists for row drivers that are able to supply a signal with a boosted voltage to a pixel array's row select, reset, and transfer gates, while not allowing the terminal voltages of the transistors in the row driver circuit to exceed the operating voltage supply.